Golf balls are generally classified as one of two construction types: wound construction and solid construction. Wound balls typically contain a solid or liquid center, elastomeric thread windings about the center, and a cover. Solid balls typically contain a solid polymeric core and a separately formed cover. Additional polymeric layers optionally may be interposed as intermediate layers between the center or core and the cover in each of the wound and solid construction types to obtain desired performance characteristics. It is generally understood in the art that solid balls may provide better initial velocity, distance and durability than wound balls, while wound balls may provide better feel and control than solid balls.
Due to the perceived inadequacy in terms of control and feel of some solid balls relative to wound balls, the art has proposed several modifications of conventional solid ball materials and construction in order to provide playability characteristics more closely resembling or even exceeding those of wound balls, while still retaining the preferred characteristics found in solid balls. Some of these proposed modifications include providing softer materials in the core, and adding a relatively soft intermediate or mantle layer over a conventional hard polybutadiene core. See, for example, U.S. Pat. No. 5,556,098. While these proposed modifications are directed to achieving better feel and control in a solid ball, they generally do so through a compromise of the distance and durability otherwise found in the unmodified solid ball. There therefore continues to exist a need for a golf ball construction type that optimizes the various performance characteristics such as durability, distance, initial velocity, control, and feel.
In terms of core materials for use in conventional solid ball construction, solid cores are typically compression molded from a slug of uncured or lightly cured elastomer composition comprising a high cis content polybutadiene and a metal salt of an .alpha., .beta., ethylenically unsaturated carboxylic acid such as zinc mono or diacrylate or methacrylate. To achieve higher coefficients of restitution in the core or to increase core weight, the manufacturer may include a small amount of a metal oxide such as zinc or calcium oxide. Other materials used in the core composition include compatible rubbers or ionomers, and low molecular weight fatty acids such as stearic acid. Free radical initiator catalysts such as peroxides are admixed with the core composition so that on the application of heat and pressure, a complex curing or cross-linking reaction occurs.
In terms of core geometry in solid ball construction, the prior art generally teaches the use of a smooth spherical core for use in conventional solid golf balls. The spherical core construction has heretofore been believed to provide adequate performance while at the same time being the most efficient shape for use in conventional golf ball manufacturing processes. There have been variations proposed, however, such as in U.S. Pat. No. 4,229,401 and related U.S. Pat. No. 4,173,345, in which the inventors disclose a core having a series of narrow shallow surface channels each lying on great circles passing through opposite pole areas of the core. The purpose of the surface channels is to prevent the formation of air pockets between the core and cover when the cover is molded onto the core. Similarly, in U.S. Pat. No. 1,558,706, the inventors disclose a core having a surface roughened by corrugations to prevent relative movement of the cover on the core during use of the ball. In U.S. Pat. No. 698,516, there is shown a gutta-percha core (A) having indentations or perforations formed thereon, and a celluloid casing (B) formed over the core. The purpose of the indentations or perforations is to "key" the celluloid casing upon the gutta-percha shell. Finally, in U.S. Pat. No. 743,105, the inventors disclose a golf ball in which the inner surface of the cover and outer surface of the core are each provided with matching projections to give the ball an improved resilient action when struck by a club. Due to the nature of these previously proposed golf ball core modifications, none is directed to improving the control and feel of a solid golf ball; they are instead directed to, respectively, improved manufacturing ability, durability, and resilience.
There therefore continues to exist a need for a golf ball that provides the distance and durability of a solid ball construction, while at the same time providing the control and feel similar to that provided by a wound ball.